The present invention relates to digital printing press apparatus and, more particularly, to digital printing press apparatus for printing separate images on both sides of the substrate.
A number of conventional printing press systems are shown in FIGS. 1-4. These systems typically include a number of printing units, designated 2 in FIG. 1. Each unit 2 includes an image bearing surface, typically a printing cylinder 4 which holds the plate with the inked image. In the course of its rotation, printing cylinder 4 transfers the image to a blanket cylinder 6. From blanket cylinder 6 the image is transferred to the upper surface 10 of a substrate 8, such as paper, under pressure provided by a pressure cylinder 12.
In monochrome printing, a single unit 2 is sufficient. Color printing systems require a number of units 2, with each unit 2 printing a single color, such as, for example, cyan, magenta, and the like, on one face 10 of the paper 8. When printing is required on both sides, 10 and 14, of paper 8, paper 8 must be turned over and re-fed into either the same press units 2, with a set of different plates, or into a similar in-line arranged printing unit 16 (FIG. 2). In either case, the in-line configurations described give the press a relatively large footprint, i.e., the press is relatively large and spread out, occupying a relatively large amount of space.
It is very commonly required, in both sheet printing and web (continuous roll) printing, to print on both sides of a substrate. To facilitate such two-sided printing, various systems, referred to generally as perfecting presses, have been developed.
As is illustrated in FIG. 3, the presses used in conventional perfecting systems have printing units 20 which include a pair of imaging/plate cylinders 22, each bearing a different image, and a pair of blanket cylinders 24 for transmitting the two images to the substrate 30. In this method printing is effected at the same time on both sides 26 and 28 of substrate 30 which may be, for example, paper sheet or web, with the pair of blanket cylinders 24 serving as each other's pressure cylinder. An improved perfecting press of the above-referenced construction is disclosed in U.S. Pat. No. 5,284,090 to Okamura et al. which is incorporated by reference in its entirety as if fully set forth herein.
Conventional digital printing presses have closely followed the established printing configurations. Conventional digital printing presses such as, the example, the Xerox Docutech, commercially available from Xerox Corp. of Rochester, N.Y., U.S.A., print only on one side of the paper and achieve a two-sided product through sequential printing methods similar to those described above.
More recently, ways have been proposed to print a plurality of images on the same substrate in sequential order but using the same imaging drum. These systems typically make use of a plurality of developing units, as can be seen, for example, in U.S. Pat. Nos. 4,860,053 to Yamamoto et al. and 5,278,615 to B. Landa et al., which are incorporated by reference in their entirety as if fully set forth herein.
For conventional four-color printing, these multipass printers are typically approximately four times slower, all other conditions being equal, than the so-called single pass color printers such as those described in U.S. Pat. No. 5,278,589 to L. Wong and 4,809,037 to Y. Sato, which are incorporated by reference in their entirety as if fully set forth herein. Should the need arise to print on both sides of the same paper sheet, the sheet with the first printed image is stored, turned over and re-fed into the machine, as is the case, for example, with the Xerox 5775 SSE digital color printer copier.
Shown in FIG. 4 is a configuration used in the Xeikon DCP-1 electronic press system, commercially available from Xeikon AB of Mortsel, Belgium, among others, for printing on both sides of the same paper web 40 using eight imaging cylinders 42.
Single pass printers as well as multipass printers which print images on both sides of the substrate require special arrangements to ensure that the images are properly synchronized or registered on the substrate. Various methods of registering the printed images have been developed. One illustrative example of such registration methods is disclosed in U.S. Pat. No. 5,278,587 which is incorporated by reference in its entirety as if fully set forth herein. It should be noted that these solutions generally work on paper sheet edges or require the positioning of special marks, which are not part of the printed image, on the substrate.
A method to improve image registration is disclosed in U.S. Pat. No. 5,280,362 to Noguchi, wherein two images pertaining to the same page are recorded at the same time on the same electrophotographic drum. The recorded images are also transferred at the same time to the paper sheet.
There is a widely recognized need for, and it would be highly advantageous to have, a digital printing press system which is capable of continuously and simultaneously (as that term is defined below) printing two different images in proper registration, one on each side of a substrate, using the same imaging cylinder.